Improve typing & Modernize middleware

[illagrenan]

Fixes #120.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.15%. Comparing base (63c8ab7) to head (3c409a3).

Files with missing lines	Patch %	Lines
django_brotli/middleware.py	86.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
+ Coverage   91.48%   92.15%   +0.66%     
==========================================
  Files           2        2              
  Lines          47       51       +4     
  Branches        4        4              
==========================================
+ Hits           43       47       +4     
  Misses          3        3              
  Partials        1        1              

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